The present invention relates to a web coating method and apparatus for maintaining a stable coating bead while coating over splices.
The production of high quality articles, particularly photographic, photothermographic, and thermographic articles, consists of applying a thin film of a coating solution onto a continuously moving substrate or web. Thin films can be applied using a variety of techniques including: dip coating, forward and reverse roll coating, wire wound rod coating, blade coating, slot coating, slide coating, and curtain coating. Coatings can be applied as a single layer or as two or more superimposed layers. Although it is usually most convenient for the substrate to be in the form of a continuous web, it may also be formed of a succession of discrete sheets.
Slide coaters have been used extensively since the 1950s in the photographic and related industries for coating aqueous photographic emulsions with relatively low viscosity (less than 100 cP). In slide coating, it is well known to start and stop coating of a moving web by means known as xe2x80x9cpick-up.xe2x80x9d In the pick-up phase, the flow of the coating liquid is established with the coater die retracted from the web. The coating liquid drains over the die edge into a vacuum box and drain. Once the flows of all the coating liquids are stabilized from all the feed slots of the slide coating die, the die and vacuum box are moved into the coating position in a rapid manner with the web moving at the desired coating speed.
Mechanical disturbances such as nicks in the die edge can cause streak-type defects to be formed in the coated article. Contamination disturbances that may cause streaking include dirt particles lodged near the coating bead, dried or semi-dried particles of coating compound, and non-uniform wetting of the contact line of the coating liquid on the coating die edge. Non-uniform wetting on the die edge, especially after pick-up, appears to be an important factor when coating fluids containing volatile solvents. For example, contamination may adhere to the front face and/or die edge of the slide coating die. That contamination may lead to a non-uniform wetting line and possible streaking of the coating compound.
The coating gap between the moving web and the coating die is typically less than about 4 millimeters (0.157 inch). Web splices, debris on, or defects in, the web in excess of the coating gap can cause serious damage to the coating die. It is common practice to retract the coating die, and break the coating bead, to permit web splices to pass through the coating gap. After the web splice passes the coating gap, the pick-up cycle must be repeated to reestablish the coating bead.
Another problem related to slide coating is contamination of vacuum ports and drains in the vacuum box when the die is retracted from the moving web (i.e., no coating bead is present) and the coating liquid is flowing freely. Contamination of the vacuum ports and drains can lead to unstable vacuum operation causing defects and eventually requiring cessation of the coating operation to clean the vacuum box and ports. This problem is exacerbated with high viscosity fluids (about 100-10,000 centipoise or greater) that contain volatile solvents that dry much faster than water (such as methyl ethyl ketone, tetrahydrofuran, or methanol).
FIG. 1 is a schematic illustration of the interface between a coating fluid 20 traversing a top surface 22 of the coating bar 24 and a moving web 26. Front face 28 of the coating bar 24 may include a durable, low surface energy portion. The low energy portion is intended to provide the desired surface energy properties to specific locations to prevent build-up of dried material. Details regarding the process of making such durable, low surface energy portions are disclosed in commonly assigned U.S. patent application Ser. No. 08/659,053 filed May 31, 1996.
When the coating bar 24 is moved into the coating position for pick-up, as illustrated in FIG. 1, a stable coating bead 30 is formed in coating gap 32 between die edge 34 and the moving web 26. The coating gap 32 is typically between 0.0254 mm and 3.81 mm. The coating bead 30 has a static wetting line 36 along the front face 28 and a dynamic wetting line 38 on the moving web 26. The pressure just under lower meniscus 40 is preferably maintained below atmospheric pressure by a vacuum box (not shown) to stabilize the coating bead 30.
If the coating process needs to be interrupted, such as when a web splice passes the coating gap 32, the coating bar 24 and vacuum box assembly can be retracted from the web 26 until resumption of the coating is desired. Retracting the coating bar 24 increases the coating gap 32. The movement of the coating bar 24, disruption of the vacuum force on the coating bead 30 and/or the increase in the coating gap 32 typically destabilizes or breaks the coating bead 30. A significant amount of web 26 may need to be advanced before a stable coating bead 30 is reestablished, resulting in wasted coating fluid 20 and web 26.
In slide coating, it is known to deckle the coating width for various reasons such as for different products and formats. Deckling often results in unwanted leakage of air into the vacuum box because the coating bead bridging the gap between the web and the front of the coating bar is typically narrower than the width of the coating bar. Leakage is more pronounced in modern die lip designs, such as square lips, that offer little resistance to air flow. Vacuum leakage into the vacuum box is particularly troublesome because it becomes difficult to maintain an adequate level of vacuum and because the excessive volume of air flow can destabilize the coating bead.
The present invention relates to a web coating method and apparatus for continuously coating over splices with a coating fluid. The present method and apparatus permit coating over splices with minimal splice generated waste by eliminating the retraction and pick-up cycle.
The apparatus includes a coating die defining a coating gap with the moving web. The coating gap is adjustable between a coating position and a splice coating position. A web guide is positioned to guide the moving web in a first direction past the coating die such that a coating bead of the coating fluid can be formed in the coating gap. A vacuum system is positioned to generate a reduced pressure condition along a lower surface of the coating die. The vacuum system defines a vacuum gap with the moving web. The vacuum gap is adjustable independent of the coating gap between a coating position and a splice coating position. A detector signals an increase in web thickness. A controller is functionally connected to the detector. The controller adjusts the coating gap and the vacuum gap to the splice coating position in response to an increase in web thickness in excess of a predetermined magnitude while maintaining a stable coating bead. In one embodiment, the coating die is a slide coating die.
In one embodiment, the vacuum system includes a vacuum box with a front seal opposite the moving web upstream of the coating gap. The front seal rotates away from the moving web in the splice coating position. In the illustrated embodiment, the web guide is a support roll. The support roll moves horizontally away from the coating gap in the splice coating position.
In one embodiment, the controller is capable of adjusting a magnitude of the reduced pressure condition in response to the detector signaling an increase in web thickness. The change in the magnitude of the reduced pressure condition preferably corresponds to the increase in web thickness reaching the coating gap. In another embodiment, the slide coating die has a die edge with a centrally located coating portion interposed between a pair of coating gap seals. The coating gap seals comprise vacuum seal land areas having a contour corresponding to a contour of the web guide.
The invention is also directed to a method for continuous coating of a moving web and splices with a coating fluid. A coating die is located opposite the moving web. The coating die defines a coating gap with the moving web in a coating position. The moving web is guided in a first direction past the coating die such that a coating bead of the coating fluid is formed in the coating gap. A reduced pressure condition is generated along a lower surface of the coating bead. An increase in web thickness is signaled to a controller. A vacuum gap is adjusted to the splice coating position in response to an increase in web thickness. The coating gap is adjusted to the splice coating position independently of the vacuum gap in response to an increase in web thickness in excess of a predetermined magnitude while maintaining a stable coating bead.